Modern computing devices typically utilize some form of pointing device for interactions of the user(s) with the Operating System (OS) or Graphical User Interface (GUI). The Capacitive Touch Pad (CTP) is well established as the pointing device of choice for Laptop and Notebook portable computers, and other devices.
A Conceptual Capacitive Touch Pad 60 is demonstrated in FIG. 6. A non-conductive cover that provides galvanic isolation between the user's hand and the Sensor is omitted for clarity here and in FIGS. 1 through 4.
Two groups of electrodes 61 and 62 are utilized. Group 61, with electrodes parallel to the X-axis, is used for determination of the Y coordinate (according to the system of coordinates depicted as 63). Group of electrodes 62, with electrodes parallel to the Y-axis, is used for determination of the X coordinate. When digit 64 is located on or near the surface of the Sensor, the capacitances 65 between the digit 64 and electrodes belonging to group 61 (as illustrated in FIG. 6) allow for the determination of the position of the digit 64 in the Y-axis. The capacitances between the digit 64 and group of electrodes 62 (not shown for clarity) allow the determination of the position in the X-axis. It should be noted that the user's body does not need a galvanic contact to ground, and parasitic body capacitance 66 to ground is sufficient for reliable operations, as it is typically several orders of magnitude larger than the capacitances 65.
It will be appreciated by a person skilled in the art, that the topology of the electrodes and their exact shape and position will greatly affect the operations of the Capacitive Touch Pad. The capacitances 65 are directly proportional to the area of contact (footprint) between the digit 64 and groups of electrodes 61 and 62. Therefore, it is desired that the areas of the electrodes on the surface of the Sensor be as great as possible.
One possible approach utilized in the Prior Art devices is shown in FIG. 7. There, the diamond-shaped elements are interconnected into Rows and Columns. The Columns 71 are created by connecting the elements by a trace on the same top layer. The Row elements 72 are joined together by traces 75 on a second layer. The Row element may have galvanic connections, through vias between the layers, or may be capacitively coupled to the traces 75. The distance between the Rows is illustrated as Spacing 73.
It should be self-evident that if a user touches the pad generating a footprint 74 with diameter less than the spacing 73 and positioned as shown, the footprint will not register on the Rows at all.
In the Prior Art implementations, the ability to sense the small targets is achieved by increasing the number of Rows and Columns. However, this approach necessitates a corresponding increase in the number of signal lines on the sensing and controlling circuit, often implemented as a single Solid-State Integrated Circuit (IC) or a group of ICs, with corresponding increase in costs.
It is much more preferable to achieve the objective of small target sensing by some other means, rather than the brute-force approach of employing a simple increase of the number of the sensing electrodes.